Heat-activatable microporous membrane and its uses in batteries

ABSTRACT

A novel microporous membrane comprising a hot-melt adhesive and an engineering plastics, the methods of preparing such microporous membrane and the uses of the microporous membrane in, e.g., batteries, super capacitors, fuel cells, sensors, electrochromic devices or the like.

FIELD OF THE INVENTION

The present invention relates generally to a microporous membrane,methods of making the microporous membrane and, in particular, to theuse of the membrane in making batteries. For the purpose of the presentinvention, a microporous membrane and a separator refer to the samestructural elements of a battery.

BACKGROUND OF THE INVENTION

Electrically-powered automotive vehicles, such as automobiles, buses andtrucks, are more environmentally friendly since they do not dischargeexhaust gases that contribute to air pollution. These vehicles areconventionally powered by a storage battery pack, which supplies theelectrical energy for operating the vehicle on the open road, includingcharging circuitry to enable recharging of the batteries such as byconnection to a conventional electrical supply. However, such vehiclesare seriously limited in the distance they can travel between batterycharges. The lack of batteries having high energy density and longbattery life is one of the major factors hindering a more widespread useof electric vehicles. Moreover, rapid growth in the wirelesscommunication market, along with the need for increased mobility andhigher power requirement, also require the development of improvedbattery technologies.

The lithium-ion battery has been the preferred power sources for variousapplications because of its higher energy density, longer cycle life ofcharging-discharging and the absence of a “memory” effect problem. Inthe early 1990's, the first liquid lithium-ion battery “LLB” wascommercialized by SONY Corporation and the worldwide market of LLB hasgrown significantly in the last ten years. In 1997, the LLB emerged asthe leader in the portable electronics market capturing a significantmarket share.

The LLB is produced mainly in a spiral wound configuration in which aseparator is sandwiched between positive and negative electrode ribbons.The separator used for LLB is a hydrophobic polyolefin based porouspolymer such as polyethylene “PE”, polypropylene “PP”, and a trilayerPP/PE/PP (U.S. Pat. Nos. 4,620,956; 5,667,911; 5,691,077). The trilayerPP/PE/PP separator developed and produced by Celgard LLC has beencommonly used in LLB production for several years.

A polymer lithium-ion battery “PLB” has also been developed for use inportable devices by replacing the liquid electrolyte with a solidpolymer or gel polymer electrolyte. Gozdz et al. U.S. Pat. No.5,418,091, May 23, 1995 and U.S. Pat. No. 5,607,485, Mar. 4, 1997disclose a plastic battery cell that is made by laminating together aseparator between positive and negative electrodes. The separatorcontains a polymer and a plasticizer, and is substantially devoid ofpores. In the process of battery assembly, after lamination at hightemperature and pressure, porosity is formed in the separator as well asin the electrodes as a result of extracting the plasticizer with avolatile solvent. As this process requires extraction of theplasticizer, it increases the potential for environmental pollution aswell as the cost of making the battery. Moreover, the current collectormaterial for electrodes, such as aluminum foil and copper foil, whichhave been commonly used for LLB batteries cannot be used for such PLBbatteries. The current collect materials of electrodes for the PLBbatteries must be in the form of grid or screen of metals such asaluminum grid and copper grid, which increases the cost of production.

Sun, U.S. Pat. No. 5,603,982, Feb. 18,1997 and U.S. Pat. No. 5,69,974,Mar. 11, 1997 disclosed the use of solid polymer electrolyte films whichare produced by in-situ polymerization of three monomers, together witha lithium salt and plasticizers in the batteries. The resulting gelpolymer electrolyte film is able to adhere to the electrodes of thebatteries when applying a vacuum to seal the battery package.

However, as the lithium salts used in Sun is sensitive to moisture, thebattery assembly operation has to be performed under anhydrousconditions, for example, in dry box under a nitrogen or an argon or indry room. This substantially increases the cost of producing these typesof batteries.

An improved gel polymer battery has been developed to reduce the cost ofproduction as this type of battery uses the same electrodes as the LLBproduct, i.e., positive and negative electrode materials coated ontoaluminum foil and copper foil respectively, and does not require dry boxfor battery assembly. A gel polymer lithium-ion battery has been made(Sanyo Corporation) by using same LLB battery electrodes, separator andliquid electrolyte with the further addition of monomers to the liquidelectrolyte and subsequently polymerizing the monomers inside thebattery case. Although the resulting gel polymer can adhere to theelectrodes, the level of adhesion is low and susceptible to separationfrom the electrodes, and the binding between separator and electrodescould be deteriorated easily during battery operation.

Pendalwar et al., U.S. Pat. No. 5,716,421, Feb. 10, 1998 discloses a gelpolymer lithium-ion battery using standard LLB battery electrodes andelectrolyte, but replacing the ordinary polyolefin type porous separatorsuch as Celgard® separator (a polyolefin-based microporous membrane)with a multi-layer coated separator which is produced by coating apolymer layer onto the polyolefin separator. However, this coatingprocess reduces the porosity of the separator as the polymer penetratesinto and clogs the pores of the porous polyolefin separator. This, inturn, reduces the charging-discharging rate capability of the battery.Moreover, the binding of the gel to the electrodes is weak and thebattery can easily be deteriorated, especially for lager sized batteriessuch as those for use in automotive vehicles.

BRIEF SUMMARY OF THE INVENTION

One aspect of the invention is directed to a microporous membranecomprising (a) a hot-melt adhesive, (b) an engineering plastics, (c)optionally a tackifier and (d) optionally a filler.

In another aspect of the invention, a microporous membrane is made by(a) dissolving hot-melt adhesive, engineering plastics, and optionally atackifier in an organic solvent, and then adding a pore former andoptionally a filler to form a homogeneous slurry, (b) casting the slurryas a film onto a support substrate, (c) evaporating the solvent from themembrane, and (d) washing the resulting microporous membrane with waterto form a microporous membrane. A preferred pore former is lithiumbromide. The resulting microporous membrane is particularly useful inthe construction of a battery, particularly a lithium-ion battery.

The contents of the patents and publications cited herein and thecontents of documents cited in these patents and publications are herebyincorporated herein by reference to the extent permitted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of battery voltage versus percent discharge capacityof a 11-cm² battery using the microporous membrane of example 2 when thebattery is discharged at various current levels.

FIG. 2 is a graph showing the percent total discharge capacity whenvarious battery cells are discharged at various current rate levels fortwo 11 cm² battery cells using the microporous membranes in example 2(circle) and example 18 (square), respectively and a 11 cm² LLB batteryusing a commercially available separator, Celgard® 2300, (triangle) forcomparison.

DETAILED DESCRIPTION

As used herein, “engineering plastics” includes, but is not limited tosynthetic thermoplastic polymers, such as condensation polymers,olefinic polymers, and blends thereof disclosed in U.S. Pat. No.4,247,498, the contents of which is incorporated herein by reference tothe extent permitted. “Engineering plastics” also includes anythermoplastic polymers having consistent and reproducible physicalproperties, which permit their use in the present invention. Preferredpolymers include polyimides, polyamide imides, polyether imides,polysulfone, polyether sulfones, polyaryl sulfones, polyether ketones,polyether ether ketones, polyphenylene sulfides, polyarylates andengineered polyamides, e.g., polyamides which have been chemicallymodified, filled or blended with another polymer to achieve the desiredlevel of physical properties, such as forming a film and beingcompatible with the hot melt adhesive material. It also includespolybutylene terephthalate, polystyrene, styrene-maleic anhydride,polychlorofluoroethane, polysulfone, polycarbonate, andpoly(styrene-methyl methacrylate). For the purpose of the presentinvention, fiberglass reinforced plastics can also be used.

“Hot-melt adhesive” includes, but is not limited to: poly(ethylene-vinylacetate) products having weight content of vinyl acetate of from about25 to about 75%, preferably from about 40 to about 70%, and morepreferably from about 40 to about 50%; and poly(ethylene-alkyl acrylate)which has weight content of alkyl acrylate from about 10 to about 30% byweight wherein the alkyl group preferably comprises from one to aboutfive carbon atoms. The hot-melt adhesives also include those disclosedin U.S. Pat. No. 4,487,891 which is incorporated herein by reference tothe extent permitted.

In a preferred embodiment, the hot-melt adhesive is present in an amountof from about 2 to about 50% by weight, preferably from about 5 to 30%,and more preferably from about 10 to about 15%.

Preferably, the hot-melt adhesive is selected from poly(ethylene-vinylacetate) and poly(ethylene-alkyl acrylate), wherein the alkyl groupcomprises from one to about five carbon atoms, more preferably, thepoly(ethylene-vinyl acetate) comprises from about 25 to about 90 weight% of vinyl acetate and from about 10 to about 75 weight % of ethylene.

In a preferred embodiment, the engineering plastic material is presentin an amount of from about 20 to about 90% by weight, preferably fromabout 40 to about 70%, and more preferably from about 50 to about 60%.Preferably, the engineering plastics is selected from the groupconsisting of polysulfone, polycarbonate, poly(styrene-methylmethacrylate), and combinations thereof; more preferably, thepoly(styrene-methyl methacrylate) has a styrene:methyl methacrylateratio of from about 9:1 to about 1:1; more preferably from about 5:1 toabout 1:1.

In a preferred embodiment, the microporous membrane further comprises atackifier, preferably in an amount from about 0 to about 50% by weight,more preferably from about 2 to about 30%, and the most preferably fromabout 5 to about 10%. The tackifier is preferably selected from thegroup consisting of hydrocarbon resin, such as Escorez® 2000 series(aromatic modified aliphatic hydrocarbon resin made by Exxon-MobileChemical Company), Escorez® 5000 series (hydrogenated hydrocarbon resinsmade by Exxon-Mobile Chemical Company), and poly(vinylidenefluoride-hexafluoropropene) “PVdF-HFP” and combinations thereof. Thepoly(vinylidene fluoride-hexafluoropropene) preferably has a weightcontent of hexafluropropene in the range of from about 5 to about 12%.

In a preferred embodiment, the microporous membrane further comprises aparticulate filler, preferably in an amount of from about 0 to about 50by weight, more preferably from about 5 to about 30%, and mostpreferably from about 15 to about 25%. The filler is preferably selectedfrom the group consisting of fumed silica, alumina, titanium dioxide,molecular sieve, calcium carbonate, calcium silicate, glass, ceramicmaterial and polytetrafluoroethylene each in the form of fine powder,and combinations thereof. Preferably the filler has an average particlesize of less than about 50 μm, more preferably less than about 25 μm,and most preferably less than about 10 μm.

The microporous membrane preferably has a porosity from about 25 toabout 75% and more preferably from about 45 to about 70%. Themicroporous membrane can be bound onto the surface of the batteryelectrodes by heat-activation using mild heat and pressure, preferablyfrom about 35 to about 125° C., more preferably from about 40 to about120° C., and most preferably from about 45 to about 90° C., for a periodof time preferably from about 0.01 to about 250 minutes, more preferablyfrom about 1 to about 60 minutes, under a pressure for theheat-activation preferably in the range from about 0.5 to about 100 psi,more preferably from about 1 to about 30 psi. Most preferably, themicroporous membrane is adhered to the electrodes upon a combination ofheat and pressure according to the above conditions.

Without intending to be bound by any particular theory of operation, itis believed that the engineering plastics provides mechanic strength forthe microporous membrane and the hot-melt adhesive serves to bind themicroporous membrane to the electrodes after heat-treatment.

“Tackifier” or “tackifying resin” includes, but is not limited topetroleum resins, such as Escorez® 2000 series (aromatic modifiedaliphatic resins having enhanced compatibility with more polar materialssuch as poly(ethylene-vinyl acetate), Escorez® 5600 series (hydrogenatedaromatic modified cycloaliphatic hydrocarbon resins), Escorez® 5300series (aliphatic resins) and Escorez® 5400 series (hydrogenatedcycloaliphatic hydrocarbon resins) each of the Escorez® resin producedby ExxonMobil Chemical Company, rosin resin, polyterpene resin, anyother polymers or copolymers which can enhance the performance ofhot-melt adhesives such as poly(vinylidene fluoride-hexafluoropropene)“PVdF-HFP” wherein the weight content of hexafluropropene is in therange of from about 5 to about 12%. Tackifier also includes thosedisclosed in U.S. Pat. No. 5,414,039, the content of which isincorporated herein by reference to the extent permitted.

As used herein, “filler” and/or “inert filler” includes, but is notlimited to silica, alumina, titanium dioxide, molecular sieve, calciumcarbonate, calcium silicate, glass, ceramic material, andpolytetrafluoroethylene each in the form of fine powder. “Filler” alsoincludes any other materials which prevent the collapse of themicropores of the microporous membrane and enhances the ionicconductivity of the microporous membrane. Some of these materials aredisclosed in U.S. Pat. Nos. 6,057,061 and 5,622,792 the contents ofwhich are incorporated herein by reference to the extent permitted.

Without intending to be bound by any particular theory of operation, itis believed that the filler or inert filler prevents the collapse of themicropores of the microporous membrane and also to enhance the ionicconductivity of the said microporous membrane

Another aspect of the invention is directed to a method of making amicroporous membrane by (a) dissolving hot-melt adhesive, engineeringplastics, and optionally a tackifier in an organic solvent, and thenadding a pore former and optionally a filler to form a homogeneousslurry, (b) casting the slurry as a film onto a support substrate, (c)evaporating the solvent from the membrane, and (d) washing the resultingmicroporous membrane with water to form a microporous membrane. The poreformer is generally a water-soluble substance, preferably an alkalinemetal halide, a granular alkaline metal sulfate, poly(ethylene glycoldimethyl ether) or dimethylformamide “DMF”. Mixtures of pore formers canalso be used. The alkaline metal halide is preferably lithium bromide.

The term “pore former” includes, but is not limited to alkaline metalsalts, such as lithium bromide, other water soluble inorganic salts,such as granular sodium sulfate, water soluble liquid polymer, such aspoly(ethylene glycol dimethyl ether) and less volatile and water solubleliquid, such as dimethylformamide “DMF” and dimethylacetate “DMA”, aswell as water soluble organic compounds in particulate form, such asstarch.

“Solvent” includes, but is not limited to a volatile organic solvent,such as cycloaliphatic ether, e.g. tetrahydrofuran “THF”; ketone, e.g.acetone and methyl ethyl ketone “MEK”; linear esters, e.g. ethylacetate; cyclic esters, e.g. gamma-butylactone; and acetonitrile.

Another advantage of the present invention is that the microporousmembrane is hydrophilic and has higher wettability with a polarelectrolyte, i.e. lower surface resistance. Consequently a higherconductivity. The membrane has a higher charging-discharging ratecapability than the conventional battery comprising separator membranewhich is hydrophobic and made of polypropylene “PE”, polypropylene “PP”,and combination of PE and PP. Moreover, the microporous membrane hashigher porosity and larger pore size which can be chemically engineeredby selecting different pore former materials having different averageparticulate ranges and in different proportions. In addition, themicroporous membrane can be bound onto electrodes with or without theaddition of an adhesive and after heat-activation results in a stronginterface between the membrane and electrodes. When the microporousmembrane is used in a battery, it offers low and stable impedance duringmany charging and discharging cycles, longer cycle life, betterperformance at high temperature, improved safety features.

Without intending to be bound by any particular theory of operation, itis believed that the binding between the microporous membrane andelectrodes is a result of physically fusing the hot-melt adhesive, andtherefore, it provides a permanent and stronger bond than that providedby the prior art gel polymer electrolyte.

A further aspect of the invention is directed to a battery comprising(1) at least one positive electrode, preferably a lithium-ion positiveelectrode, (2) at least one negative electrode, preferably a lithium-ionnegative electrode, (3) an electrolyte, preferably a lithium-ionelectrolyte and more preferably a liquid lithium-ion electrolyte or apolymer lithium-ion electrolyte, and (4) a microporous membranecomprising (a) a hot-melt adhesive, (b) an engineering plastics, (c)optionally a tackifier and (d) optionally a filler.

As used herein, the terms “battery cell” and “cell” are usedinterchangeably. The term “battery” means either one battery cell ormultiple battery cells.

An important utility for the microporous membrane is in themanufacturing of rechargeable lithium-ion batteries, whereinconventional electrode materials can be used to make the positive andthe negative electrodes. Preferably, the positive electrode is made withlithiated metal oxide such as lithium cobalt (III) oxide “LiCoO2”,lithium nickel oxide “LiNiO₂”, lithium manganese oxide “LiMn₂O₄” andcombination thereof. The negative electrode is preferably made of carbonin a form such as coke or graphite. However, any electrode materialsknown in the art can be used herein.

The invention additionally relates to a method of manufacturing themicroporous membrane and rechargeable lithium-ion battery. The inventionfurther relates to the use of this microporous membrane in batteries,super capacitors, fuel cells, sensors, electrochromic devices or thelike.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples. All parts andpercentages in the examples, as well as in the remainder of thespecification, are by weight unless otherwise specified.

Further, any range of numbers recited in the specification or paragraphshereinafter describing or claiming various aspects of the invention,such as that representing a particular set of properties, units ofmeasure, conditions, physical states or percentages, is intended toliterally incorporate expressly herein by reference or otherwise, anynumber falling within such range, including any subset of numbers orranges subsumed within any range so recited. The term “about” when usedas a modifier for, or in conjunction with, a variable, is intended toconvey that the numbers and ranges disclosed herein are flexible andthat practice of the present invention by those skilled in the art usingtemperatures, concentrations, amounts, contents, carbon numbers, andproperties that are outside of the range or different from a singlevalue, will achieve the desired result, namely, a microporous membraneand method for preparing such membrane as well as a battery comprisingthe membrane.

EXAMPLE 1

A microporous membrane was prepared as follows: 0.90 g ofpoly(ethylene-vinyl acetate) containing 50 wt. % of vinyl acetate,“PEVA-50”, 0.36 g of Escorez® 2596 (aromatic modified aliphatichydrocarbon resin) made by ExxonMobil Chemical Co. of Houston, Tex., and4.14 g of polysulfone (Mw. 80,000) made by Scientific Polymer ProductsInc. of Ontario, N.Y., “PSU”, were dissolved in 40 g of tetrahydrofuran“THF”. To this solution, was added 1.00 g of fumed silica. The mixturewas stirred overnight. Then, to this slurry, was added 7.00 g of lithiumbromide, “LiBr”. After the LiBr was dissolved completely, the slurry wascast directly onto 4 inch by 15 inch pieces of Mylar® (polyethyleneterephthalate) film substrate at room temperature. The coating thicknesswas controlled at about 200 μm. Ten pieces of membrane films were madewith this slurry. THF evaporated from the membrane at room temperaturein about half an hour. The resulting membrane was soaked in water for anhour as soon as THF evaporated completely. Then, the membrane was washedwith fresh water thoroughly to obtain a microporous membrane bycompletely removing the LiBr. Finally, the microporous membrane wasallowed to be dried at room temperature. The thickness of the drymicroporous membrane was measured to be about 1 mil thick, i.e. 0.001inch. The microporous membrane films were found to be uniform,homogeneous and mechanically strong. Data relating to this example issummarized in Table 1 as Sample No. 1.

EXAMPLE 2

This example is shown in Table 1, Sample No.2 and was made using thesame slurry as described in Example 1 except with a substitution ofEscorez® 5690 (cycloaliphatic hydrocarbon resins) made by ExxonMobilChemical Co. of Houston, Tex., in place of for the Escorez® 2596(aromatic modified aliphatic resins).

EXAMPLE 3

Sample No. 3 was made using the same slurry mixture as for Example 2except with a substitution of poly(ethylene/vinyl acetate) containing 70wt. % of vinyl acetate, “PEVA-70”, for PEVA-50.

EXAMPLE 4

In this Example, a microporous membrane was prepared as follows: 0.90 gof poly(ethylene-vinyl acetate) containing 50 wt. % of vinyl acetate,“PEVA-50”, 0.36 g of Solef® 20810 (poly(vinylidenefluoride-hexafluoropropene) containing 8% of hexafluoropropene, made bySolvay Advanced Polymer Inc. of Houston, Tex.), and 4.14 g ofpolysulfone were dissolved in 40 g of tetrahydrofuran “THF”. To thissolution, was added 1.50 g of fumed silica. The mixture was stirredovernight. Then, to this slurry, was added 7.00 g of lithium bromide“LiBr”. After the LiBr dissolved completely, the slurry was castdirectly onto 4 inch by 15 inch pieces of Mylar® film substrate at roomtemperature. The coating thickness was controlled at around 200 μm. Theresulting membrane was soaked into water for an hour as soon as THFevaporated completely in about half an hour. Then, the membrane waswashed with fresh water thoroughly to obtain a microporous membrane.Finally, the microporous membrane was dried at room temperature. Thethickness of the dried microporous membrane was measured to be 1.2 milthick, i.e. 0.0012 inch. The microporous membrane films were found to beuniform, homogeneous and mechanically strong. Data relating to thisexample is summarized in Table 1 as Sample No. 4. Additional data suchas pore size and porosity content for this Sample No.4 are recorded inTable 2.

EXAMPLES 5-11

As summarized in Table 1, these seven samples, identified as Sample Nos.5 through 11, were microporous membranes made with the same slurrydescribed in Example 4 except the ingredients were present in differentconcentrations. Sample No. 6 was made without the use of inert filler,while Sample No. 9 was made without the use of tackifier.

EXAMPLES 12-13

In Table 1, Sample No. 12 and 13 are microporous membrane films madewith the same slurry mixture as described in Example 4 except for Sample12 with a substitution of alumina (average particle size less than about10 μm) made by Sigma-Aldrich Inc of Milwaukee, Wis., for silica, whilefor Sample No. 13, with a substitution of molecular sieve (averageparticle size less than about 10 μm) made also by Sigma-Aldrich Inc. forsilica and in the absence of tackifying resin.

EXAMPLE 14

In this example, microporous membrane Sample No. 14 was made with thesame slurry described in Example 4 except with a substitution of PEVA-40for PEVA-50, a second substitution of Solef® 21216 (poly(vinylidenefluoride-hexafluoropropene) containing 12% of hexafluoropropene) forSolef® 20810, and a third substitution of poly(bisphenol-A carbonate)“PBAC”, for polysulfone.

EXAMPLE 15

In Table 1, Sample No. 15 is a microporous membrane made with the sameslurry described in Example 14 except with a substitution ofdimethylformamide “DMF” for LiBr and with the use of PEVA-40 indifferent concentration, but without the use of inert filler.

EXAMPLE 16

In this example, microporous membrane Sample No. 16 was made with thesame slurry described in Example 14 except with a substitution ofpoly(methyl methacrylate), “PMMA”, for PBAC and with substitution ofmethyl ethyl ketone “MEK” for THF, and in the absence of tackifyingresin. The microporous membrane obtained was found to be brittle and hadreduced strength.

EXAMPLE 17

In this instance, microporous membrane Sample No. 17 was made with thesame slurry described in Example 14 except a substitution ofpoly(styrene-methyl methacrylate) in ratio of 70:30, “PSMMA”, forpolysulfone.

EXAMPLE 18

This microporous membrane Sample No. 18 was made with the same slurrydescribed in Example 4 except a substitution of RTP #905 which is apolysufone resin containing 30% glass fiber for reinforcement made byRTP Company of Winova, Minn., for pure polysulfone. Data relating tothis sample are recorded in Table 1 and 2.

TABLE 1 Microporous membrane composition and proportions (g) PoreResulting Sample Hot-melt Eng. Inert Solvent former microporous No.adhesive Tackifier Plastic filler (g) (g) membrane 1 PEVA- Escorez ® PSUSilica THF LiBr uniform 50 2596 4.14 1.00 40 7.00 0.90 0.36 2 PEVA-Escorez ® PSU Silica THF LiBr uniform 50 5690 4.14 1.00 40 7.00 0.900.36 3 PEVA- Escorez ® PSU Silica THF LiBr uniform 70 5690 4.14 1.00 407.00 0.90 0.36 4 PEVA- Solef ® PSU Silica THF LiBr uniform 50 20810 4.141.50 40 7.00 0.90 0.36 5 PEVA- Solef ® PSU Silica THF LiBr uniform 5020810 3.96 1.00 40 7.00 0.90 0.54 6 PEVA- Solef ® PSU (None) THF LiBrPEVA 50 20810 3.96 40 7.00 distributed 0.90 0.54 unevenly 7 PEVA-Solef ® PSU Silica THF LiBr uniform 50 20810 3.96 1.50 40 7.00 0.90 0.548 PEVA- Solef ® PSU Silica THF LiBr uniform 50 20810 4.14 1.00 40 7.000.90 0.36 9 PEVA- (None) PSU Silica THF LiBr PEVA 50 4.50 1.00 40 7.00distributed 0.90 unevenly 10  PEVA- Solef ® PSU Silica THF Na₂SO₄ notuniform 50 20810 4.14 1.S0 40 7.00 0.90 0.36 11  PEVA- Solef ® PSUSilica THF PEGDE- uniform 50 20810 4.14 1.50 40 250 0.90 0.36 7.00 12 PEVA- Solef ® PSU Alumina THF LiBr uniform 50 20810 4.14 1.00 40 7.000.90 0.36 13  PEVA- (None) PSU M. Sieve THF LiBr uniform 50 4.05 1.00 407.00 0.63 14  PEVA- Solef ® PBAC Silica THF LiBr uniform 40 21216 1.001.00 40 5.00 0.63 2.50 15  PEVA- Solef ® PBAC (None) THF DMF uniform 4021216 2.00 40 5.00 1.00 2.50 16  PEVA- (None) PMMA Silica MEK LiBrbrittle, 40 2.50 2.00 40 5.00 reduced 0.63 strength 17  PEVA- Solef ®PSMMA Silica THF LiBr uniform 40 21216 4.05 1.00 40 5.00 0.63 0.45 18 PEVA- Solef ® RTP Silica THF LiBr uniform 50 20810 #905 1.00 40 7.000.90 0.36 4.14

In Table 1, the abbreviations are as follows: PEVA-50,poly(ethylene-vinyl acetate) containing 50 wt. % of vinyl acetate;PEVA-40, poly(ethylene-vinyl acetate) containing 40 wt. % of vinylacetate; PEVA-70, poly(ethylene-vinyl acetate) containing 70 wt. % ofvinyl acetate; PSU, polysulfone; PBAC, poly(bisphenol-A carbonate);PMMA, poly(methyl methacrylate); PSMMA, poly(styrene-methylmethacrylate); M. Sieve, molecular sieve; THF, tetrahydrofuran; MEK,methyl ethyl ketone; PEGDE-250, poly(ethylene glycol dimethyl ether)with molecular weight of 250; DMF, dimethylformamide.

EXAMPLE 19 Characterization of the Microporous Membrane

Some of the microporous membrane samples made as described above weresubjected to characterization including thickness, basis weight, mean ofpore size, and porosity. Data relating to Sample Nos. 4, 8, 11, 12 areset forth in Table 2. The commercial product Celgard® 2300 were alsotested as a control and are set forth in the same table.

The medium value of pore size of microporous membranes as recorded inTable 2 was determined by porometry. Each of these four samples havemuch higher porosity than Celgard® 2300. It is believed that higherporosity of the microporous membrane can result in highercharging-discharging rate capability of battery.

TABLE 2 Basis Thickness Weight Pore Size % Sample No. (inch) (g/m²) (μm)Porosity 4 0.0012 13.36 0.43 69.6 8 0.0012 16.46 0.57 61.0 11 0.000915.16 0.44 54.0 12 0.0011 16.33 0.56 64.8 Celgard ® 2300 0.0010 13.850.46 39.9

The microporous membrane is different from the hydrophobic polyolefintype separator in at least the following respects: a) improvedhydrophilicity, i.e. good wettability with a polar electrolyte resultingin a lower surface resistance; b) improved microporous quality; c) thepore size as well as porosity of the microporous membrane can bechemically engineered; c) the membrane is heat activatable havingimproved electrode contact and adhesion.

EXAMPLE 20 Preparation of Batteries

A lithium-ion rechargeable battery was assembled using a carbon negativeelectrode, a LiCoO₂ positive electrode, and a membrane of Sample No. 2.Both negative and positive electrodes were conventional liquidlithium-ion battery electrodes, namely negative electrode containingabout 90% active carbon material, and the positive electrode containingabout 91% active LiCoO₂.

A microporous membrane of Sample No. 2 with a dimension of 38 mm by 45mm was sandwiched between a positive electrode 30 mm by 38 mm and anegative electrode 32 mm by 40 mm, i.e. the battery having a totalactive area of 11 cm². The battery was packaged and partially sealed inan aluminum foil-laminated plastic bag. After the battery was fullydried, it was transferred into a dry-box under nitrogen and having lessthan 1 ppm moisture. About 0.4 g of 1.2M electrolyte was injected intothe battery, wherein the electrolyte was prepared by dissolving LiPF₆salt produced by Stella Chemifa Corp. of Osaka, Japan, into a solutionof ethylene carbonate/diethyl carbonate/dimethyl carbonate 2:1:1. Thebattery was completely sealed, and then was heated at a temperature of85° C. for 30 minutes and then was pressed at a pressure of 5 psi tobind the microporous membrane to the positive and the negativeelectrodes. After cooling the battery down to room temperature, thebattery was subjected to charge and discharge testing.

FIG. 1 shows voltage versus capacity when discharged at various currentlevels for this battery cell. The discharge current rate was 0.2 C, 0.5C,1 C, 1.5 C and 2 C respectively, i.e. the current density fordischarge was 0.6, 1.5, 3.0, 4.5, and 6 mA/cm² respectively. Performancedata relating to this battery cell is summarized in Table 3 as BatteryNo. 1.

At the end of performance test, the battery was disassembled and themicroporous membrane used for the battery was found to be firmly boundonto the positive and negative electrodes.

A second battery was assembled using a piece of microporous membraneSample No. 18 in the same manner as described above for Battery No. 1.Testing data of the resulting battery are recorded in Table 3 as BatteryNo. 2.

For comparison, a third battery was made in the same way as describedabove for Battery No. 1 except with a substitution of conventionalseparator Celgard® 2300 for microporous membrane Sample No. 2 as BatteryNo. 3 (control battery). For charge and discharge performance test, thecontrol battery was held between two plates to assure the proper contactbetween separator and electrodes because the Celgard® separator isunable to be bound to electrodes even through heat activation/treatment.Testing data on the control battery are also recorded in Table 3 asBattery No. 3.

The discharge-recharge cycle performance results of these threebatteries are also shown in FIG. 2. The figure shows percent totaldischarge capacity when the batteries are discharged at various currentrate levels for batteries No. 1 (circle) and 2 (square), and alsoBattery No. 3 (triangle) for comparison. Both batteries Nos. 1 and 2offered higher discharging rate capability than the control battery.

TABLE 3 Rate Battery Microporous Rate capability capability at No.membrane at 1C rate (%) 2C rate (%) Battery 2 97.1 77.9 No. 1 Battery 1897.1 73.6 No. 2 Battery Celgard ® 2300 96.7 71.6 No. 3

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art, withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A microporous membrane having at least one layer which comprises (a) a hot-melt adhesive, (b) an engineering plastics, (c) a tackifier and (d) a filler; wherein the engineering plastics is present in an amount of from about 20 to about 90% by weight.
 2. The microporous membrane of claim 1, wherein the membrane can be bound to battery electrodes by heat-activation at a temperature of from about 35 to about 125° C. and under a pressure of from about 0.5 to about 100 psi.
 3. The microporous membrane of claim 1, wherein the microporous membrane has a porosity from about 25 to about 75%.
 4. The microporous membrane of claim 1, wherein the membrane has a porosity of from about 45 to about 70%.
 5. The microporous membrane of claim 1, wherein the hot-melt adhesive is poly(ethylene-vinyl acetate) or poly(ethylene-alkyl acrylate).
 6. A microporous membrane having at least one layer which comprises (a) a hot-melt adhesive, (b) an engineering plastics, (c) a tackifier and (d) a filler; wherein the hot-melt adhesive is poly(ethylene-vinyl acetate) or poly(ethylene-alkyl acrylate); wherein said alkyl group comprises from one to about five carbon atoms.
 7. A microporous membrane having at least one layer which comprises (a) a hot-melt adhesive, (b) an engineering plastics, (c) a tackifier and (d) a filler; wherein the engineering plastics is selected from the group consisting of polysulfone, polycarbonate, poly(styrene-methyl methacrylate), and combinations thereof.
 8. A microporous membrane comprising (a) a hot-melt adhesive, (b) an engineering plastics, (c) a tackifier and (d) a filler; wherein the engineering plastics is selected from the group consisting of polysulfone, polycarbonate poly(styrene-methyl methacrylate), and combinations thereof.
 9. The microporous membrane of claim 8, wherein the poly(styrene-methyl methacrylate) comprises a styrene:methyl methacrylate ratio of from about 9:1 to about 1:1.
 10. The microporous membrane of claim 1, wherein the tackifier is selected from the group consisting of aromatic modified aliphatic hydrocarbon resins, aliphatic resins, hydrogenated cycloaliphatic hydrocarbon resins, rosin resin, polyterpene resin, PVdF-HFP, and combinations thereof.
 11. The microporous membrane of claim 10, wherein the content of hexafluropropene in PVdF-HFP is from about 5 to about 12%.
 12. The microporous membrane of claim 1, wherein the filler is selected from the group consisting of fumed silica, alumina, titanium dioxide, molecular sieve, calcium carbonate, calcium silicate, glass, ceramic material and polytetrafluoroethylene and combinations thereof.
 13. The microporous membrane of claim 12, wherein the filler is in the form of powder having an average particle size of less than about 25 μm.
 14. The microporous membrane of claim 13, wherein the filler has an average particle size of less than about 10 μm.
 15. The microporous membrane of claim 1, wherein the hot-melt adhesive is present in an amount of from about 2 to about 50% by weight.
 16. The microporous membrane of claim 1, wherein the tackifier is present in an amount of from about 2 to about 30% by weight.
 17. The microporous membrane of claim 1, wherein the filler is present in an amount of from about 5 to about 30% by weight.
 18. A microporous membrane comprising (a) a poly(ethylene-vinyl acetate) or a poly(ethylene-alkyl acrylate), wherein said alkyl group comprises from one to about five carbon atoms, (b) a polysulfone, a polycarbonate, a poly(styrene-methyl methacrylate) or a combinations thereof, (c) a tackifier and (d) a silica.
 19. The microporous membrane of claim 18, wherein the poly(ethylene-vinyl acetate) or the poly(ethylene-alkyl acrylate) is present in an amount of from about 10 to 15% of the membrane by weight, wherein the polysulfone, the polycarbonate, the poly(styrene-methyl methacrylate) or the combinations thereof is present in an amount of from about 50 to about 60%, wherein the tackifier is present in an amount of from about 5 to about 10%, and wherein the silica is present in an amount from about 15 to about 25%. 